The invention relates to a sintering device for a blank of foamable, prefoamed plastics. The device comprises mould walls separable from each other. Between the walls is formed a mould cavity in communication with a controllable injector for the plastics. Passages in communication with chambers which surround these mould walls are provided and to which process steam (optionally sub-pressure) and/or compressed air can be controllably supplied. In the cavity there is arranged at least one mould core which consists of a fixed jacket of controllably changeable form.